1. Field of the Invention
The present invention relates to an LED lamp for emitting an approximately equal amount of light in all directions except backward from the base of the LED lamp.
2. Description of the Related Art
Conventionally, there have been devices for displaying characters and symbols to give information to people at a distance using small light emitting bulbs. These devices can be, for example, a news board of a limited size on which a stream of characters inform passengers of various news items. Around 1930, small filament lamps were put into practical use for these news boards. With the improvements in the technology for the display control of such display devices, the small filament lamps have come to be used for a large-size monochrome animation screen for outdoor use. However, most of the present large-size outdoor screens have display elements of cold-cathode tubes.
Since the small filament lamps can emit light forward, sideward, and obliquely backward, that is, in all directions except backward from the base of the bulb where electrodes are mounted, they are used, for example, as the illuminations on Christmas trees, which can be seen from every direction.
Recently, in addition to the above described small filament lamps, LED lamps are used as light emitting elements with an LED element embedded in a transparent resin or glass bulb-shaped portion as shown in FIG. 1. FIGS. 1A, 1B, and 1C are a front view, a side sectional view, and a bottom view respectively of the LED lamp. As shown in FIGS. 1A through 1C, the LED lamp comprises an epoxy resin bulb-shaped portion 3 formed with a flange 2 incorporated into its base; two leads 4, one end of each is extended outside the bulb-shaped portion 3 and the other end is embedded in the bulb-shaped portion 3; and an LED chip 5 embedded in the bulb-shaped portion 3 and connected to the ends 3 of the two leads 4.
One LED element is provided for the LED chip 5. If the luminous energy of the LED lamp should be increased, the number of the LED chips 5 embedded in the bulb-shaped portion 3 is increased corresponding to the desired luminous energy. Increasing the luminous energy to a certain extent can also be realized by increasing the bias voltage applied to the LED element. Generally, the diameter .phi. of the bulb-shaped portion 3 of the LED lamp is approximately 3 mm through 5 mm, and 10 mm at maximum.
Normally, the light emitted from the LED element is very directional. For example, FIGS. 2A through 2E show the standard directional characteristics of the light emitted from an LED element at various emission angles. As shown in FIG. 2E, the emission angle is 10.degree. or smaller in the forward direction. On the other hand, as shown in FIG. 2A, the emission angle is 80.degree. or smaller in the forward direction.
Therefore, the LED lamp is not suitable for applications where light should be emitted in all directions like illuminations on a Christmas tree. Normally, the LED lamp is used for a display screen of a device on which information can be read from the front, such as a time table board at a station, a flight information board at an airport, etc.
Recently, a push button (or a touch button) 6 as shown in FIG. 3A is adopted as a stop button in an elevator hall, a control button in an elevator, etc. The above described LED lamps are also used for such push buttons because of their durability and energy-saving properties.
In this case, however, the light emission range 1a is narrow when there is only one LED lamp as shown in FIG. 3B. As a result, when the push button 6 is viewed from the front, only the center portion 6a of the push button 6 appears luminous as shown in FIG. 3C. Therefore, in practice, a large number of LED lamps 1 (15 in FIG. 4) are mounted to an LED matrix 7 as shown in FIG. 4. An optically-diffusing plate 8-1, a legend plate 8-2, and a push button plate 6 are sequentially mounted on the front of the LED matrix 7, and they are contained in a housing 8-3 to be used as an LED unit, that is, for example, a push button.
Assume that, as shown in FIG. 5, the LED lamp 1 is used for a light source 9 which hemispherical and needs to be recognized from many directions, such as a warning light on a fire hydrant provided at a subway station, a subway, etc., an emergency light at an emergency exit in an office building, a hotel, etc. A number of LED lamps 1 are mounted across the base and toward the flange of the light source 9, and should be arranged in such a way that the central axis of each of the LED lamps 1 is assigned at a progressively varied angle from the center toward the flange. With the thus-arranged LED lamps, the light can be emitted from the hemispherical light source 9 in all directions.
Thus, the LED lamp has been used for radially emitting light from a bulb-shaped portion.
However, since the above described filament lamps and cold-cathode tubes can be easily broken even when receiving only a small shock, because their bodies are made of thin glass bulb- or tube-shaped portions, they therefore require very careful handling and can give a lot of trouble to users. Furthermore, they are inconsistent in structure and luminous characteristics and have a relatively short operating life, thereby giving users the trouble of frequently replacing faulty bulb- and tube-shaped portions.
Furthermore, since such news boards require an enormous number of light emitting bulbs, the small filament lamps are not economical because each of the small filament lamps has a relatively high power consumption. On the other hand, the cold-cathode tube has the demerit in structure that it cannot form a small picture element of a screen like the small filament lamps.
The LED element also has the problem that it is limited in usage because it is directional in optical-emission as described above, although it is durable and consistent in emission characteristics. Furthermore, to obtain a light diffusing in all directions using the LED element, a great number of LED elements are required or an optically-diffusing board must be provided.
Additionally, a time table board at a station and a flight information board at an airport are also required to be seen from all directions, in order to allow the users to recognize the existence of the time table board or the flight information board from the side of the boards, even if they cannot correctly read the displayed characters, etc.